Why

CODEX_RS_SSE_FIXTURE let integration-style CLI, exec, and TUI tests bypass the normal Responses transport by reading SSE from local files. That kept test-only behavior wired through production client code. The affected tests can stay hermetic by using the existing core_test_support::responses mock server and passing openai_base_url instead.

What Changed

• Removed the CODEX_RS_SSE_FIXTURE flag, codex_api::stream_from_fixture, the env-flags dependency, and the checked-in SSE fixture files.
• Repointed the affected core, exec, and TUI tests at MockServer with the existing SSE event constructors.
• Removed the Bazel test data plumbing for the deleted fixtures and refreshed cargo/Bazel lock state.

Verification

• cargo build -p codex-cli
• cargo test -p codex-api
• cargo test -p codex-core --test all responses_api_stream_cli
• cargo test -p codex-core --test all integration_creates_and_checks_session_file
• cargo test -p codex-exec --test all ephemeral
• cargo test -p codex-exec --test all resume
• cargo test -p codex-tui --test all resume_startup_does_not_consume_model_availability_nux_count
• just bazel-lock-update
• just bazel-lock-check
• just fix -p codex-api -p codex-core -p codex-exec -p codex-tui
• git diff --check